Optical sensors (e.g., fiber optic sensors) are used in various applications due to their unique properties, such as small size, dielectric structure, electromagnetic immunity, robustness, chemical durability, high sensitivity, and cylindrical geometry. These properties make optical sensors especially suitable for applications in extreme environments, biomedical applications, and in all fields of industrial application. They may be used to measure almost any physical or chemical parameter such as strain, temperature, pressure, acceleration, refraction index, displacement, and others. Many optical sensors may be designed to change their spectral characteristics under the influence of a measured parameter. For example, a sensor may change a peak wavelength value within a reflected characteristic spectrum, when exposed to variations of the measured parameter. Such optical sensors may be, for example, fiber Bragg gratings or Fabry-Perot interferometer based sensors. The measured parameter (e.g., strain, temperature, pressure, acceleration, refraction index, displacement, and others) in these types of optical sensors may be extracted by acquisition, and the processing of their spectral characteristics.
Various systems and methods for interrogating optical sensors have been researched and described. Interrogation, as used herein in its broadest sense, means to determine one or more spectral characteristics of an optical sensor. However, prior systems generally require relatively complex electronic and optical subsystems to accomplish sensor interrogation. One of the most direct and commonly used systems for the determination of measured parameters from the spectral characteristics of fiber optic sensors is based on using a wide-band optical source of about 15 nm to 100 nm or more and an optical spectrum analyzer. Light generated by the wide-band optical source travels through an optical fiber to the optical sensor. The optical sensor then reflects back part of the optical spectrum, which is then captured by the optical spectrum analyzer. The optical spectrum analyzer acquires the spectral characteristics of the back-reflected light and searches for specific features within the spectral characteristics, such as peak wavelength within the back reflected optical spectrum, for example. This peak wavelength, thus, may provide information about a measured physical or chemical parameter, i.e., when the optical sensor is exposed to a physical or chemical input.
In general, the main disadvantage of systems employing spectral analysis is in the complex structure of the optical spectrum analyzer, especially when high resolution wavelength measurements are desired, which is the case in most practical optical sensor applications.
Somewhat simpler sensor interrogation systems may be built by application of narrow-band optical sources with a tunable wavelength. In one prior system, for example, measuring pressure using a Fabry-Perot interferometer, and temperature using a Bragg grating sensor, may be based on wavelength sweeping of a tunable laser source in combination with an optical time-domain reflectometer (OTDR). Short pulses of different wavelengths may be sent to the optical sensor. From time differences between the reflected signals, it may be possible to reconstruct the spectral characteristics of each individual optical sensor. However, such prior systems based on OTDR are generally complex, expensive, and inappropriate for interrogating small numbers of sensors.
Other prior art systems utilize wavelength control of a tunable optical source during wavelength sweeping. However, such tunable optical sources are complex elements. They can be made either from a broad-band optical source and tunable narrow-band filter or by using more complex technology such as laser sources with a wavelength tunable optical feedback, such as tunable optical resonator. Both versions can provide a widely tunable wavelength range of between about 10 nm and 300 nm. However, the complexities and associated high costs of such sources may only justify their application in systems containing a large number of sensors. Furthermore, the wavelength of the narrow-band tunable optical sources can drift with aging. This drift may cause long term instabilities. For this reason, these kinds of systems use an additional external wavelength reference for precise determination of optical source wavelength. Such references help in wavelength determination of an optical source, but they also exhibit temperature dependence, which may cause errors when measuring a physical parameter with the optical sensor. Of course, use of such reference units may limit the minimum size of the system, add reliability issues, and increase the complexity and cost of the system. Their use is thereby justified in larger and more complex sensor networks, while such an approach is generally inappropriate for a single sensor or signal interrogations with a low number of channels.
For the above-described reasons, there is a long felt and unmet need for highly integrated and compact optical sensor interrogation methods, systems, and apparatus.